Farnborough Air Show

Europe on cutting edge of noise, vibration research

 - November 15, 2006, 7:16 AM

Noise and vibrations continue to be the focus of several research projects in Europe. For instance, the pan-European Sefa and ABC projects are targeting external aircraft noise and passenger comfort on airliners and helicopters, respectively. French-funded Dyna is trying to better understand engine aeroelasticity and behavior under impact. Several manufacturers, as well as research institutes, are involved in these programs.

Sefa (Sound Engineering for Aircraft) is looking at why, at a particular sound level, some aircraft noises disturb more people than others living near airports. As part of this project, French aerospace research firm Onera is creating a “virtual resident” that would be able to assess aircraft noise perception beginning in the design phase.

The project is seeking to identify several psycho-acoustic factors. “These are noise characteristics that can trigger a psychological reaction,” Jean-Louis Gobert, a researcher at Onera’s long-term design and systems integration department, told Aviation International News. The characteristics can include the roughness of the noise, how fast the volume increases and how it varies. Gobert explained that some relatively low-range noises–produced by landing gear, flaps, and the like, as well as by engines–will appear more bothersome than louder ones, depending on psycho-acoustic factors.

Onera asked a group of 300 Europeans to participate in a study assessing their perceptions of various aircraft sounds–at takeoff, landing and so forth. To avoid bias, none of the study participants lives near an airport. Researchers are linking their perceptions of a given noise to a database that Onera is using to develop a software program. The goal of the program is to be able to work out a virtual perception of new sounds, yielding a virtual sample of how the population would actually experience the noise. This, in turn, would give aircraft manufacturers a tool to improve an aircraft’s perceived noise level without having to decrease the actual volume of the noise.

“If you cannot lower a noise, you may move its frequency upward or downward,” Gobert explained. Car manufacturers have been using this principle for a long time to reduce the impact of noise.

The $7.5 million Sefa project is part of the European Commission’s Sixth Framework research program, which started in 2004 and will end next year. The project leader is Germany’s Dornier GmbH, and participants include companies such as French engine maker Snecma (Hall 4 Stand B12), aerospace European giant EADS (Hall 4 Stand G16) and Italian-based aircraft manufacturer Alenia (Outside OE2).

At its transonic wind tunnel, Onera recently completed a measurement program that was part of the helicopter active blade concept (ABC) research project. Researchers studied rotor blades fitted with piezoelectric actuators that move flaps on the trailing edge. With adequate control laws, the actuators can reduce vibrations during descent. Still to be determined is whether they reduce noise as well.

The program demonstrated the efficiency of the design against the vibratory level transmitted by the rotor to the fuselage. According to Onera, performance of the rotors remained unchanged. The actuator could be displaced by 0.5 millimeter, which was converted into trailing edge rotation–plus or minus five degrees. Control surfaces were deflected at frequencies up to 80 Hz.

The researchers are pursuing two other objectives. The first is to see whether active blades can reduce energy consumption in high-speed cruise. The second is to evaluate whether such control surfaces could replace a conventional cyclic swash plate. Test data is being processed to determine how these can be achieved.

Bucking Convention

In the wind tunnel, the 14-foot diameter rotor model faced specific demands. For example, its smaller size (scale was one third) implied a higher rotation speed, up to 1000 rpm. Therefore, acceleration at blade tip was much higher–2,000 g versus 600 g–than in reality. Hence the need for a flexible metal leaf, versus a conventional hinge like the one used at full scale.

Series production of a helicopter equipped with such active blades is unlikely in the near future. “We are talking about 2015,” said Hugues Mercier des Rochettes, a blade design engineer at Onera. On a real-life rotor, the main issue will probably be maintenance, he added. Usually, blades are maintenance-free because they are passive.

During the wind tunnel work, few of the tests involved noise measurement. Late this year, further tests should help understand whether these active control surfaces can limit the noise. They will take place in one of the wind tunnels operated by the DNW German-Dutch consortium, which now has a formal alliance with Onera.

All these wind tunnel tests complement the flight tests that began on an experimental BK 117 rotorcraft last fall. The ABC project started in 1998 and involves Onera’s German counterpart, DLR, and Eurocopter (Hall 4 Stand G16).

With the Dyna project, Snecma, Onera and helicopter engine manufacturer Turbomeca (Hall 4 Stand B12) are just about to start aeroelasticity and vibration tests on a turbomachine. Dyna’s first objective is to better understand vibratory phenomena, as engine specifications become more demanding. The second objective is to study the behavior of the structure under impact–with a bird, ice, stones or other foreign objects.

Onera will begin tests on a turbomachine (not a complete engine) equipped with accelerometers and optical devices. “We will excite the turbomachine body in order to identify it in a rather high frequency band. During the second phase, and according to the results, we plan to continue our investigations in an even higher frequency band,” explained Jean-Pierre Grisval, coordinator of this project for Onera.

Dyna, funded by the French civil aviation authority DGAC, was launched in 2004 and
is now halfway into its four-year term.

The Cost of Bad Design

Aeroelasticity and vibration problems are a major cause of uncertainty in engine design. For the engine manufacturer, economical consequences can be tremendous. Delays resulting from aeroelasticity problems can last for many months.

Flutter risk mainly relates to engine fans. The trend toward very-high-bypass-ratio powerplants has driven blade design to wide-chord, shroudless airfoils. Because of this, entry into an instability zone can have more serious consequences. Hence the need for identification and evaluation of these instability zones beginning at the design stage.

The other aeroelasticity problem for engine manufacturers is the response to various aerodynamic excitations. On the fan, these excitations are due to some nonuniformity of the flow in the air intake. On the other stages, these excitations come from upstream airfoil wakes.

The conventional method is to try to avoid any frequency interaction when at steady regimes. Taking into account all the frequency interaction permutations has proved to be an insurmountable problem. Statistically, when a multi-stage compressor is being developed, designers encounter at least one vibration or aeroelasticity problem. Moreover, manufacturers have been continuously endeavoring to reduce the number of fan stages and to make them more compact. This trend is increasing excitation levels.

Quite recently, developers have used computational stability software programs in engine design. However, these methods are kept simple enough to be compatible with today’s computers capabilities. These tools cannot cope with the complex physics of these phenomena in the entire engine operation envelope.

With Dyna, researchers hope to learn much more about turbomachine aeroelasticity and vibrations. This should help blade design and might also improve active blade control.

Much, too, has to be learned about fast dynamics under impact. Researchers still have to do some modeling on how serious the impact itself is. They also have to better understand the possible resulting loss of a blade. “One objective is to develop tools to anticipate blade behavior under impact,” Grisval said. Improved anticipation may limit the amount of tests in an engine certification program.